1. Technical Field
The present invention relates to a cavity filter, more particularly to a cavity filter that includes a ceramic resonator.
2. Description of the Related Art
With advances in mobile communication, there have been rapid increases in demand for RF equipment such as filters, duplexers, multiplexers, and the like. RF equipment may be used in the filtering, separation, and transfer of signals in places such as base stations, etc., in a mobile communication system.
An RF filter is a device for passing the signals of a particular frequency band. In devices that require high power, such as in the base station of a mobile communication system, a cavity filter having a cavity-based structure is mainly used.
A cavity filter, the structure of which may include multiple cavities formed within the filter with resonators installed inside the cavities, is a filter that performs the filtering by way of resonance in each of the cavities.
One of the most frequently used resonators in a cavity filter is the coaxial resonator, which is structured to have a cylindrical form with a hole or recess formed therein.
With respect to mobile communication systems, there is a demand for transmission and reception performance of higher sensitivity, as well as a demand for smaller equipment. In particular, along with the increase in number of low-output compact base stations, there is a growing demand is for smaller sizes in equipment used in such base stations. As such, there is a continuous demand for smaller sizes also in the cavity filter using coaxial resonators.
In the past, resonators of a step-impedance structure were used, in which the shape of the coaxial resonator was changed to enable a smaller size for the coaxial resonator cavity filter.
FIG. 1 conceptually illustrates a cavity filter using a resonator of a step impedance structure according to the related art.
Referring to FIG. 1, the cavity filter using a resonator of a step impedance structure may include a housing 10, a resonator 20, and a cover 30.
As can be seen in FIG. 1, a cavity filter that uses a resonator having the step impedance structure has the shape of the resonator 20 modified at its upper end from the existing cylindrical form. By thus forming a step impedance section to increase the gap capacitance between the cover 30 and the resonator 20, the resonance frequency could be lowered to enable a smaller size of the resonator 20.
However, such modification of the shape of the coaxial resonator can no longer satisfy the demands for smaller size as required in current base stations.